In recent years, in view of social requirements for low fuel consumption and low exhaust emission, a hybrid vehicle having an engine and a motor generator (MG) as a vehicle power source attracts attention. In the hybrid vehicle, as disclosed in, for example, JP-A-9-287490 (corresponding to U.S. Pat. No. 6,007,443), the MG is provided on a power transmission channel to transmit engine power to a driving shaft of the vehicle, and a transmission for automatically changing a transmission gear ratio in correspondence with MG rotation speed is provided between the engine and the MG. The fuel consumption is improved by controlling the engine rotation speed and the torque to constant values.
However, according to the above-described technique, because the MG rotation shaft and the vehicle driving shaft are always linked, it is difficult to drive the MG to generate power with the engine power during the vehicle stop. Further, in the worst case, there is a possibility that the vehicle cannot keep running due to reduction of a battery residual power. When a clutch is provided between the MG and the driving shaft as a countermeasure against the above-described difficulty, because it is necessary to control the clutch with high accuracy at vehicle starting, it is difficult to ensure a responsiveness of a driving torque. Thus, a starting performance may be deteriorated.
Japanese Patent No. 3,461,652 (corresponding to U.S. Pat. No. 6,007,443) discloses that a MG is driven to generate power with engine power during vehicle stop, by linking an engine output shaft, a MG rotation shaft and a vehicle driving shaft via planetary gear unit.
However, in the above-described technique, because the engine output shaft, the MG rotation shaft and the vehicle driving shaft are linked via the planetary gear unit, the structure of a power transmission system may be complicated or enlarged.